In an optical communications system, information is encoded onto a light signal. The light signal is transmitted from one point to another, as for example by free-space light beams or optical fibers. At the receiving end, the information is read from the light signal.
An important advantage of optical communications is that a number of different light signals of different wavelengths may be mixed together (multiplexed) into a single light beam in a technique known as wavelength division multiplexing (WDM). Each light signal of a different wavelength, or channel, has information encoded onto it prior to the mixing of the channels. At the receiving end, the channels are separated, or demultiplexed, according to their wavelengths. The information on each channel is read from the demultiplexed light of that wavelength. A single multiplexed light beam may therefore carry many times the information that may be transmitted by a non-multiplexed light beam.
The demultiplexing may be accomplished using a series of light bandpass filters. A thin-film optical filter of this type is formed as a substrate and a multilayer dielectric light-transmissive optical stack deposited upon the substrate. The bandpass filter transmits only light of a specific wavelength. When a multichannel beam is incident upon the filter, the light channel associated with the bandpass range is transmitted through the filter to a light receiver behind the filter. The beam with the remaining channels is reflected to another filter, where the next channel is extracted from the beam in a similar fashion, and so on until all of the channels of information carried by the light beam have been separated for further individual processing.
As the number of channels transmitted on a light beam increases, the required number of different constructions of the demultiplexing bandpass filters increases in order to extract each of the different channels from the light beam. The demultiplexing structure therefore becomes increasingly more costly due to the large numbers of different types of light bandpass filters that must be constructed and tested. There is accordingly a need for an approach which accomplishes the demultiplexing in a more convenient and less costly manner, particularly for large numbers of channels carried on the light beam. The present invention fulfills this need, and further provides related advantages.